With an EN40B crank you would have no concerns as the case depth of that crank's hardening process is quite deep. The EN16T (T for Tufftride) has a shallower case depth but still is in the .008 to .015 range, depending on material and hardening time. Since you will only be taking .005" off the surface for a .010" regrind you should be OK and still retain the hardness close to the original.
Having said that, from what I have seen over the years, the Brits were not very disciplined in terms of quality control and basically assembled the cars with whatever came down the line. I have never seen this but I am told there were times when they didn't heat treat the cranks at all. To guard against that you could take a hardness reading in a non-critical place like in line with an oil hole in the middle of a journal before and after the grind and you will know for sure.
Good luck with your LP1275 project. As I have told you many times it is the way to go. I am loving the ease in starting, the longer time between valve adjustments, and the long time between rebuilds due to the life of the rings, bearings, and valve train.
